U.S. Pat. No. 3,979,709, Sept. 7, 1976 to Daniel P. Healey, Jr. relates to an electric fuse whose casing is manufactured by the so-called pultrusion process. by using that process as described in the above patent tubular casings may be obtained which have a great dynamic strength, i.e. a great strength to resist impulse loads as occurring in fuses, particularly current-limiting fuses, and which comply - so it seemed - with all other requirements of tubular fuse casings, or fuse tubes. It was, however, discovered that the mass consisting of glass fibers and cured polyester, of which pultruded tubing of the aforementioned kind is made, has a larger heat-transmitting conductivity than conventional fuse tube materials, such as, for instance, vulcanized fiber or convolutely rolled glass-cloth melamine. When using the tube material described by Healy U.S. Pat. No. 3,979,709, in order to obtain the same current rating as in an existing fuse, all fusible elements had to be redesigned or re-rated, or the wall tubing had to be given a thickness exceeding mechanical strength requirements. None of these solutions is economically acceptable.
It is well known to line fuse tubes with a layer of asbestos where a reduction of the radially outward heat flow from the fusible element, or elements, is desired. But asbestos is a carcinogen, i.e. a cancer-causing substance, and was ruled out for this reason from the very outset.
This invention is the outgrowth of a systematic investigation of materials which are non-carcinogenous, and are capable - though in form of thin lining sheets - to impart to pultruded glass cloth polyester tubings as described in Healey U.S. Pat. No. 3,979,709 about the same thermal conductivity as conventional fuse tube materials, e.g. glass-cloth-melamine laminates.
The product which meets the above requirements is a tough, paperlike product including highly purified asbestos in non-carcigenous form, and other insulation materials manufactured and sold by Johns-Manville under the trademark "QUINTEX II".